The invention relates to a flow control valve, and in particular, to a flow control valve having a drooping response for the flow rate, namely, the response in which the flow rate supplied to a hydraulic instrument decreases with an increase in the discharge flow rate from a pump.
A flow control valve which exhibits such response is generally used in a power steering apparatus for vehicles and are useful in providing the stability to the vehicle when running at high speeds and in reducing the horsepower dissipated. An apparatus of this kind is disclosed, for example, in Japanese Laid-Open Patent Applications No. 104,186/1981 and No. 4,469/1982 in which an orifice is defined in a supply passage which feeds hydraulic fluid discharged from a pump to a hydraulic instrument, with a pressure differential across the orifice being effective to open a spool valve to return part of the hydraulic fluid, and in which a restriction is defined in the supply passage so that a control spool which is responsive to a pressure differential across the restriction operates to reduce the area of the orifice.
In an arrangement in which a pressure differential across the restriction is utilized to control the reduction in the area of the orifice, such pressure differential acts as an internal pressure loss within the pump, thus militating against the reduction in the horsepower dissipated as the flow rate decreases. Accordingly, a proposal is made which prevents the pressure differential across the restriction from increasing beyond a value required (see Japanese Laid-Open Patent Applications No. 146,472/1981 and No. 4,470/1982).
However, the flow control valves described above employ a complex construction and an increased number of parts, which must be machined to a high precision. In addition, difficulty is experienced in turning operating response.